Modern vehicles use three-way catalysts (TWC) for exhaust after-treatment of gasoline engines. With tightening government regulations on automobile emissions, air-fuel ratio of each engine cylinder in a multi-cylinder engine is closely monitored. Ideally, each engine cylinder should be designated with an exhaust gas composition sensor for accurately measuring the air-fuel ratio of the cylinder. However, due to affordability constraints, in practice, exhaust gas oxygen sensors positioned upstream and downstream of the TWC may be used to control the air-fuel ratio near stoichiometry.
Other attempts to address cylinder imbalance include detecting and mitigating air-fuel ratio imbalance based on a pre-catalyst and a post-catalyst sensor. One example approach is shown by Yoshikawa et al. in U.S. Pat. No. 8,695,568 B2. Therein, an air-fuel ratio control unit performs main air-fuel ratio control based on the output of the pre-catalyst sensor and auxiliary air-fuel ratio control based on the output of the post-catalyst sensor. The inter-cylinder imbalance is further detected based on the variation of engine speed.
However, the inventors herein have recognized potential issues with such systems. As one example, the pre-catalyst and post-catalyst sensors may have different sensitivity to the air-fuel ratio of each engine cylinder due to zoned exhaust flow. Especially in naturally aspirated engines, the physical geometry and arrangement of engine cylinders create a non-uniform, zoned exhaust flow condition in the exhaust system. Due to monolithic catalyst designs that may be used in some examples, zoned exhaust flow is preserved through the catalyst. Various faults, such as an air-fuel ratio imbalance between cylinders, may exacerbate this non-uniform, zoned exhaust flow condition so that neither the pre-catalyst nor the post-catalyst sensor may equally detect the exhaust gas concentration from all of the cylinders. As such, the air-fuel ratio control performed in U.S. Pat. No. 8,695,568 B2 may not successfully mitigate the imbalance.
In one example, the issues described above may be addressed by a method for identifying and mitigating cylinder imbalance, comprising: identifying an imbalanced cylinder based on each of a first sensor positioned upstream of a catalyst and a second sensor downstream of the catalyst, wherein the first and the second sensors are positioned on opposite sides relative to a central axis of an exhaust passage; and adjusting an air-fuel ratio of the imbalanced cylinder based on a magnitude of the fault via fuel injectors. In this way, the imbalanced cylinder may be identified utilizing the zoned exhaust flow. Further, air-fuel ratio in the imbalanced cylinder may be corrected without extra measurement.
As one example, cylinder-to-cylinder imbalance may be detected based on the outputs of a pre-catalyst sensor and a post-catalyst sensor. The pre-catalyst sensor is positioned on opposite sides relative to a central axis of the exhaust passage. In response to identifying the imbalanced cylinder, the magnitude of fault in air-fuel ratio is calculated for the imbalanced cylinder. The amount of fuel injection to the imbalanced cylinder is then corrected based on magnitude of the fault. As such, air-fuel ratio information of both individual cylinder and multiple cylinders in a cylinder bank may be measured and controlled. In this way, instead of mitigating the zoned exhaust flow for improved air-fuel ratio control, zoned exhaust flow is accepted and utilized to determine and subsequently correct cylinder-to-cylinder imbalance.
The technical effect of identifying an imbalanced cylinder with two sensors positioned on opposite sides relative to the center line of the exhaust passage is that the air-fuel ratio of each cylinder may be accurately detected by different sensors. The technical effect of positioning a pre-catalyst sensor upstream of the catalyst and a post-catalyst sensor downstream of the catalyst is that the total number of exhaust gas oxygen sensors used in the engine system may be minimized. The technical effect of adjusting the air-fuel ratio of the imbalanced cylinder based on the outputs of the pre-catalyst and the post-catalyst sensor is that the cylinder-to-cylinder imbalance may be mitigated without acquiring extra information about the engine system. As such, the current method simplifies the detection and mitigation of cylinder-to-cylinder imbalance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.